Fix threading issue of Atlas.FallbackStack, and introduce overloads with fallback parameter

[Saplonily]

This pr changes the Atlas.FallbackStack implementation into ThreadLocal<T>, since the expected usage of it is that, pushing a fallback, get something from Atlas in this context, and then pop it, which make sense to make it thread local.
If we're using ConcurrentStack, then one thread may get a fallback that pushed from another thread, which is not expected.

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There's an optional commit that marks stack operation methods as obsolete, and add some overloads that accept a fallback parameter, because this is a much easier way to reach that.

namespace Monocle;

public class Atlas
{
+   [Obsolete(MessageFallbackStackObsolete)]
    public void PushFallback(MTexture fallback)

+   [Obsolete(MessageFallbackStackObsolete)]
    public MTexture PopFallback()

+   public MTexture GetDefaultFallback();
+   public MTexture GetAtlasSubtexturesAt(string key, int index, MTexture fallback)
+   public List<MTexture> GetAtlasSubtextures(string key, MTexture fallback)
+   public MTexture GetWithFallback(string id, MTexture fallback) 
}